Hmmm.. I know it makes for compelling viewing, in terms of revenge, and there's something very satisfying about seeing characters who are screwing your favourite characters over get screwed over in turn. But in terms of it being morally acceptable, that's a grey area... I mean, Willow killing was seen as unacceptable, mostly because he was a human, personally my issues have less to do with that (but that's another discussion) and more to do with "Chasing him down, premeditating the killing, keeping him alive, torturing him, and ripping his skin off" part.
I don't know, I guess it varies... I can actually understand Wood- as someone said, his mother was a Slayer, as in killer-of-vampires. A vampire murders his mother, I can understand why he'd want to kill that vampire, if he got the chance. He knows what they're capable of, and what killers they are. And, y'know, Spike wasn't exactly all cuddles and blunt edges around Season 7, lest we forget- though I do agree that he had a soul, and so there is a line between Spike the purposeful-killer, and Spike the souled being manipulated by the First. Wood, however, once again, grew up with the "Vampires are evil and Spike killed my mother on purpose"- much like Angelus's actions tainted the Scooby Gang's reaction to ensouled Angel, I guess. It wasn't the finest motive in the world, and it wasn't the best time for it, but I can understand it.
And with the mention of Angelus... one piece of revenge I have no moral qualms about supporting wholeheartedly- Giles beating the crap out of Angelus with the aid of a Molotov cocktail, and a flaming baseball bat after he kills Jenny. Possibly because Passion is a wonderful episode, and breaks my heart, perhaps because he screws up my OTP, possibly because it is a genuinely kick-ass Giles moment (Bear in mind that Angelus is a legendary vampire who survived for centuries, and Giles is a human man in his late 40s, and Giles beats the crap out of him with a flaming baseball bat), possibly because the one thing that annoys me in mid-late Season 2 is that, until Jenny's death, Buffy can't bring herself to kill Angel, and the rest of the Scooby gang aren't trying to finish him off- OK, I can see Willow maybe being against it for Buffy's sake, but are we really supposed to buy that Giles thinks Buffy being sad about Angel dying is more important than the lives that are being lost as a result of leaving him to frolick about??? OK, sorry, rant over...
Anyway, yeah, that scene comes on, I am cheering Giles on, and at the same time, getting irritated that every time I watch it, he insists on pausing him beating to go for a swinging shot and give Angelus time to recover... "Noooo, you fool! 😡" So I suppose in some cases, yeah, I can definitely get behind the revenge. In real life, however, a lot more of a grey area.